The overall goal of the proposed studies remains to understand the mechanisms of ischemic kidney cell injury and repair with a long range goal to establish therapies that will be useful to prevent and treat acute kidney injury (AKI) in man. Over the last support period we have further clarified the role of leukocyte- endothelial interactions, identified anti-inflammatory docosanoids which may hasten recovery, identified testosterone's contributions to the pathophysiology of AKI, and developed and validated a new biomarker of proximal tubule injury (KIM-1) in rodents and man. We have developed a new model of toxic injury in zebrafish, established and characterized mouse models of preconditioning and found that bone-marrow derived stem cells do not play a direct role in the reconstitution of the epithelium post ischemia. In the current proposal the first Specific Aim addresses the source of cells that depopulate the proximal epithelium during repair and the potential contribution of proximal tubule cells to the population of tissue phagocytes and fibroblasts that participate in normal and abnormal repair after ischemia. Genetic techniques will be used to label proximal tubule cells or their precursors either during development or later. We will evaluate whether stem/progenitor cells participate in repair or whether repopulation of the epithelium occurs as a result of dedifferentiation and proliferation of surviving epithelial cells? Using genetic labeling approaches in the second and third parts of this specific aim we will determine whether macrophage-like cells or fibroblasts derive from proximal tubule cells or proximal tubule epithelial progenitors after ischemia. In the second Specific Aim we will develop a new genetic model of targeted damage to the proximal tubule epithelial cell or macrophages using cell specific expression of the Diphtheria toxin receptor (DTR), and will use this approach to establish the determinants of the inflammatory and preconditioning responses and explore the role of macrophages in the pathophysiology of ischemia/ reperfusion injury and repair. Relevance (Lay summary): Kidney disease affects a large segment of the population. It is important to understand the relative roles of stem/progenitor cells and mature cells in the normal and abnormal repair of the kidney after it is injured. In addition the use of novel genetic approaches to target injury to the kidney will help in the determination of the factors important for "good" vs "bad" repair short term and long term.